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Time-dependent density functional theory calculations of X-ray absorption



There has been dramatic progress in recent years both in calculations and in the interpretation of X-ray absorption spectra (XAS). Often an independent-electron approximation with final state potentials is adequate. However, for soft X-rays (i.e., energies less than about 1 keV) local field effects can be important. Such local fields arise from the dynamic screening of both the external X-ray field and the coupling to the core hole created in the absorption process. These effects require a theory that goes beyond the independent-electron approximation. We developed an efficient approach for treating such effects in molecules and solids based on a generalization of time-dependent density functional theory (TDDFT), with a local approximation for the screening response. The approach has been implemented in our self-consistent, real-space Green's function code FEFF8 in terms of screened dipole transition matrix elements. Typical results are discussed for the XAS of the N4,5 edges of solid Xe and for the L2,3 edges of 3d transition metals. Our approach accounts for the deviations of the L3/L2 intensity branching ratio from the 2:1 value of the independent electron approximation. For the N4,5 edges of Xe, the approach also accounts for the observed fine structure. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003